MLA

Kilpatrick, Jack Frederick (1915-1967). Business letter. Box 8, folder 714. April 22. https://collections.library.yale.edu/catalog/15493045.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 8, folder 714"])(April 22)[Business letter]. https://collections.library.yale.edu/catalog/15493045.